1. Field of the Invention
The present invention is directed generally to a multi-phase control or calibration system and, more particularly, to such a system exhibiting relatively constant partial pressures with respect to certain diverse dissolved gaseous species of interest in one or more liquid phases over a range of ambient temperatures. The preferred fluid consists of a first non-aqueous liquid phase containing an amount of dissolved oxygen (O.sub.2) in which the partial pressure of O.sub.2 is relatively temperature insensitive over an ambient temperature range of interest, a second, aqueous phase, immiscible with the first phase and containing an amount of dissolved carbon dioxide (CO.sub.2) and one or more solute species that provide temperature stability with respect to the partial pressure of CO.sub.2 over the ambient temperature range of interest. The liquid phases are in equilibrium with a vapor phase.
2. Description of the Related Art
Relatively inert fluids which have the ability to dissolve rather large amounts of oxygen and which are stable and do not affect biological media, for example, are known. The class of fluorinated organic compounds known as perfluorocarbons are the best known examples of such materials. Perfluorocarbon substances are completely fluorinated organic compounds in which all the carbon-bound hydrogen atoms are replaced with fluorine atoms. These materials have an unique combination of properties. The compounds are extremely non-polar and have essentially no solvent action. They are so chemically inert and have such high thermal stability that they can be mixed with almost any material without fear of any adverse reactive effect either upon other mixture components or upon the material itself. The compounds also have a relatively high boiling point and low pour point giving them a relatively wide liquid range. Many of these perfluorocarbon materials also have a high, relatively stable, oxygen solubility.
These properties have led to the use of perfluorocarbon solutions as oxygen carriers and as controls for O.sub.2 sensors. Aqueous perfluorocarbon multi-phase emulsions have been proposed for applications which involve the need to carry or sense oxygen in the presence of carbon dioxide and/or where pH needs to be controlled or sensed. These include medical related technologies involving blood and synthetic blood materials and blood gas analysis controls and calibrators. One such emulsion system is illustrated and described in U.S. Pat. No. 4,722,904 to Feil. Other such systems are disclosed in Turner (U.S. Pat. No. 4,001,142), Cormier, et al. (U.S. Pat. Nos. 4,299,728 and 4,369,127) and Sorenson, et al. (U.S. Pat. Nos. 4,116,336 and 4,151,108).
These fluids typically comprise an aqueous emulsion of the organic oxygen carrier. Certain of these emulsions may contain surfactant materials, pH buffers and preservative materials. The aqueous phase and the perfluorocarbon phase are chemically compatible but completely immiscible. While the perfluorocarbon phase reversibly carries the oxygen of interest, the aqueous phase reversibly carries other constituents of interest such as carbon dioxide and hydrogen ions.
The use of such materials, particularly as quality controls for blood gas analyzers, for example, requires that the control system contain a known partial pressure of oxygen and a known partial pressure of carbon dioxide and be of a known pH. Accordingly, the control system must be supplied in gas-tight, sealed ampules, or the like containing known amounts of dissolved oxygen and carbon dioxide species so that equilibrium partial pressures remain constant. Such a known or control substance can later be used to check the relative accuracy of an instrument utilized to measure O.sub.2 and CO.sub.2 concentration and the pH of such substances as blood.
Although such prior approaches have been successful with respect to achieving proper quality control of such devices, the integrity of the partial pressure of the dissolved species used in control measurements depends on the complete isolation of the control system from the time it is prepared until the time of use. In addition, because of the variation in solubility of the gas species of interest with temperature, the opening and use of the control must occur at a specific temperature; and thus the results are also quite temperature dependent.
There remains a definite need to reduce the sensitivity of such control systems to fluctuating or uncertain ambient temperature conditions. This would lead to more versatile uses of the materials and to the development of accurate device calibration methods without the need for rigorous environmental control at the time of calibration. Accordingly, it is an object of the present invention to develop a calibration/control system which is less temperature sensitive than known fluids of the class including aqueous emulsions of perfluorocarbons.